when the car is at a stop engine running gas gauge read correctly volt meter about 12 volts even when I rev engine to 3 to 4 thousand rpm but when the car is moving forward or reverse any speed the gas gauge goes to full and the volt meter drops to 8 volts charging system check out fine at advance auto, but I put new alternator in anyway still doer the same thing 1983 gl brat 4 speed with weber carb conversion when the car is not moving gas gauge seems to read ok an volts go up to 12 temp gauge reads fine tach ok clock ok please help.  voltage regulator is in the alternator. Thanks Ken

I would check the main dash ground located under the fusebox.

A voltmeter hooked between here and battery negative should show close to 0.0 volts even w/the blower fan and headlights on.

(and while moving)

Check all of the grounds actually.

 

Basically nothing more than, breaking the screw/bolt holding the ground, loose,, and re-tightening it helps.

 

Also, just un-plugging/re-plugging connectors help. Any and all you can find, just do it. It does make a difference.

 

Found problem on all 5 BRAT's I've owned with the fuel gauge reading funky was the connections at the tanks sending unit. Major corrosion there.

Have just cut out the bad, soldered the wires together on those. Your method may very.